Crystal and mesophase structure of a bicyclohexyl cyano mesogen

The phase behaviour of 4-[trans-4-(trans-4-propyl­cyclo­hex­yl)cyclo­hex­yl]benzo­nitrile, C22H31N, 1, has been examined. This com­pound has two different solid phases, denoted I and II, and exhibits thermotropic liquid-crystalline behaviour, with a remarkable inter­val of stability of the mesophase between the lower melting solid phase (75 °C) and the isotropization tem­per­ature (247 °C). The crystal and mol­ecular structures of solid phase I have been determined at 173 K. The cyclo­hexyl rings both adopt the chair conformation and are equatorially substituted. The packing of 1 in the crystalline state is driven by the anti­parallel arrangement of cyano dipoles with the formation of close contacts involving the strong cyano acceptor and weak aromatic C—H or aliphatic C—H donors. The crystal packing is discussed and com­pared with X-ray dif­frac­tion data in the liquid-crystalline state. The combination of thermal analysis, optical polarizing microscopy and X-ray dif­frac­tion analysis suggests that the mesophase is a partially ordered smectic phase. The lamellar structure of the mesophase is retained in crystalline solid phase II obtained by cooling the liquid-crystalline phase.